1. Field of the Invention
The invention generally relates to a portable device for testing a liquid sample, and more particularly to a multi-channel device for optically testing at least the turbidity, free chlorine, total chlorine and color of surface water, drinking water, processed water or the like.
2. Description of Related Arts
Water, intended for human use and consumption, should be free of harmful chemicals and disease-causing bacteria or other microorganisms. A chlorine gas or a chlorine solution is added to water for disinfection and control of microorganisms. Testing a residual chlorine concentration after water treatment is a very important step because chlorine is known to react with organic matter in the water to form trihalomethanes (THMs), a suspected carcinogen. Free chlorine is defined as the concentration of residual chlorine in water present as dissolved gas (Cl2), hypochlorous acid (HOCl), and/or hypochlorite ion (OCl—) Combined chlorine is defined as the residual chlorine existing in water in chemical combination with ammonia or organic amines which can be found in natural or polluted waters. Total chlorine is the sum of free and combined chlorine. Two other important parameters usually tested for drinking water are turbidity and color. Turbidity itself has no health effects, however, it can interfere with disinfection and provide a medium for microbial growth. Turbidity may indicate the presence of disease causing organisms. Color is most commonly caused by dissolved organic matter, but it may be produced by dissolved mineral matter. All those parameters: free chlorine, total chlorine, turbidity and color are the most required for routine water evaluation.
Turbidity and color can be measured using optical methods. Chlorine (free and total) can be measured using calorimetric methods when specific chemicals changing their color in chlorine presence are added to water sample and chlorine concentration can be evaluated by intensity of color produced in those chemicals. Another method also can be used to measure chlorine in water (electrochemical, ion chromatography and others).
For other applications it might be needed to measure higher levels of color, turbidity and chlorine than limits specified for drinking water. On the current market there is no such portable meter, which has a high sensitivity to measure low levels and a large range to measure high levels of analytes.
There are several commercially available turbidimeters, such as Hach 2100P Portable Turbidimeter, LaMotte 2020 Portable Turbidimeter, and combinations of turbidimeter with a colorimetric chlorine meter, such as Hanna Instruments C 114 Turbidity and Chlorine Meter. They only provide several separate meters for evaluate different parameters of drinking water, rather than any color channel integrally formed and functioning with turbidity channels. They also fail to provide performance for testing water according to current regulations, such as EPA 180.1 or ISO 7027 standard.
There are two standard specifications for turbidity measurement that are generally in use worldwide. These are the International Standard ISO 7027 (Water quality—Determination of Turbidity, International Standard, Third Edition, 1999-12-15) and the USEPA 180.1 (Nephelometric Method 2130 B, Standard Methods for the Examination of Water and Wastewater, 1989). Both methods measure the intensity of light scattered at 90 degrees to the path of incident light. The specification of the ISO standard is more stringent and requires the use of a monochromatic light source.
Hach 2100P Portable Turbidimeter requires a big sample volume (15 ml), but supports only a small turbidity range (1000 NTU maximum). In addition, it is inconvenient to use since it needs a special oil to prepare sample vials for testing of low turbidity water. LaMotte 2020 Portable Turbidimeter also supports a small turbidity range (1100 NTU maximum) with a low accuracy for a low turbidity level. Hanna Instruments C 114 Turbidity and Chlorine Meter supports an ever smaller turbidity range (only 50 NTU maximum) with a low accuracy for a low turbidity level, as well as a small chlorine range.
There are many patents, such as U.S. Pat. Nos. 3,994,590 , 4,312,593 , 4,797,900 , 5,083,868, 5,872,361, which describe methods and devices for measuring turbidity, color, or chlorine. Each of them has only one channel for measuring one parameter, rather than multiple channels for measuring multiple parameters.
U.S. Pat. No. 6,404,500 describes a multi-channel colorimetric device. This device has multiple light emission diodes (LEDs) and a radiation absorption cell. It provides a dual-purpose detector 29, i.e., a color detector+turbidity scatter channel in FIG. 5. However, most commercially available LEDs have manufacturing variations such that their various emitting intensities result in direct deviation in output absorption signals. The multi-channel colorimetric device thus has a low stability. In addition, its mathematics used for output signal evaluation was developed for out-dated, low memory, and low computational power microcontrollers. Moreover, the process involves complicated normalized absorption equations, which are not so convenient, strait forwarded and fast as look-up tables with polynomial interpolation or direct polynomial interpolations from multiple calibration points. One of its embodiments, which converts radiation absorption data into a digital wave form, has a very low sensitivity and bad time response for low intensity optical signals when frequencies of output signals are as low as 1-5 Hz . It also fails to answer how to measure an optical signal, which produce output less than 1 Hz, for a time period as short as 100 milliseconds. This is a typical problem in measuring low turbidity or high absorption in real time conditions.
U.S. Pat. No. 6,836,332 describes an instrument for testing fluid characteristics. It has multiple channels with LEDs and photovoltaic detectors for measuring multiple parameters, such as spectral transmittance, turbidity and fluorescence. However, its design has low sensitivity and stability. As mentioned, LEDs have unstable emitting intensity, spectral distribution, and spatial distribution of emitted radiation. It fails to employ reference detectors or focusing members to compensate such instability.
U.S. Pat. No. 6,844,934 describes an optical design of turbidity which includes two emitters, two detectors, a transparent cylindrical tube with liquid and lenses between emitters and the transparent cylindrical tube. The turbidity sensor in FIG. 6 has one light source channel, one scatter-signal detector, and one reference/direct signal detector placed 90 degrees from each other. However, it only applies focusing properties in a horizontal plane. In the tube section which is perpendicular to the tube section as shown on the FIG. 4, there is no focusing means and focusing properties evaluation is not valid. In addition, the optical turbidimeter does not comply with EPA 180.1 as it does not use any tungsten lamp as a light source. It also does not comply with ISO 7027 because there was no focusing member on the detector side to form an aperture angle between 10° degree. and 20° degree. in the water sample. Moreover, its big size is not convenient or portable.
Currently, there is no portable sensor for measuring color, turbidity and chlorine in water against standards specified for drinking water with high sensitivity to measure low levels and a wide range of analyses.